The number of worldwide telephone subscribers is substantial and rapidly increasing. Each such subscriber is generally connected to a telephone central office via a metallic pair of wires identified as the telephone "local loop." This huge network of communications paths is grossly under utilized as it is normally used only for sporadic voice communications between the telephone subscriber, the telephone central office, and other telephone subscribers. In addition, there is a substantial and rapidly growing demand for data access to telephone subscribers in order to provide such services as interactive t.v. viewing, at-home shopping and banking services, monitoring alarm conditions in the home or office, and a host of other applications in which it would be desirable to transmit data to, and receive data from, the telephone subscriber. The known under utilization of the telephone local loop network, and the growing demand for access to the telephone subscriber, has sparked a search for means to utilize the telephone local loop for information transfer, while leaving the underlying voice communication undisturbed.
Systems now exist which permit the simultaneous transmission of both voice and a secondary information signal over the local loop. One such system is described in U.S. Pat. Nos. 4,493,948 and 4,528,422, assigned to The Inteleplex Corporation, the same assignee to which is assigned the instant invention. Such systems generally comprise an information terminal coupled to the ring/tip conductor pair at a first location, such as the location of the telephone subscriber, and a companion information terminal coupled to the same ring/tip pair at a second location, such as the telephone central office. The information terminals are designed to transmit complementary signals, (both in terms of phase and amplitude) on the ring and tip conductors between a respective conductor and earth ground. These signals are carefully balanced longitudinally between the tip and ring conductors, and when precisely balanced, the signals will not disturb the normal underlying voice communications. It is, of course, understood that voice communication is not disturbed because the telephone equipment works on the principal of detecting a difference in voltage and/or phase between the tip and ring conductors, which makes perfectly balanced signals essentially invisible to the telephone equipment.
Systems, such as those described in U.S. Pat. Nos. 4,493,948 and 4,528,422, provide excellent service in permitting a secondary information signal to be superimposed over normal voice communications carried on the local loop. However, in order for such systems to operate properly, perfect signal balance must be maintained, at all times, between the tip and ring conductors.
A problem arises in maintaining balanced signals when transmitting over metallic wire pairs of different lengths, with differences in manufacturing tolerances, and various environmental conditions that can change the impedance of either of the conductors in the pair. Any such change in impedance, even very small changes, results in unbalanced longitudinal signals (differences in phase and amplitude) and a resultant unwanted interference (cross-talk) with voice communication being carried over the local loop.
Previous solutions to this problem have included the use of manually adjustable potentiometers within the local loop that require periodic adjustment by a technician. However, the maintenance expenses alone, associated with a circuit that must be manually adjusted would render it impractical for wide spread use, such as in conjunction with the existing telephone network.
It is, therefore, an object of the instant invention to provide method and apparatus for balancing longitudinal signals being transmitted over a metallic wire pair such as the telephone local loop.
It is a further object of the instant invention to eliminate the need to manually adjust the varying impedance of each pair of conductors in a metallic wire pair such as the telephone local loop, when transmitting longitudinal signals over the local loop.
It is a still further object of the instant invention to provide simple and relatively inexpensive equipment to automatically balance longitudinal signals being transmitted over a metallic wire pair such as the telephone local loop.